Electroless metal and metal alloy plating electrolytes usually comprise one or more sources of the metal(s) to be deposited in an ionic form, reducing agents, complexing agents, pH modifiers, accelerators and one or more stabilizer additives. The stabilizer additives stabilize such plating electrolytes against various manifestations of undesired plateout. In real life plating electrolytes, usually a mixture of several stabilizer agents is used at once in order to reach the desired electrolyte stability. Understanding their optimal replenishment rate is key to successful operation of electroless plating electrolytes. The stabilizer additives are usually employed at low concentrations, typically 1 to 100 ppm. Rapid changes of the chemistry of an operating electroless plating electrolyte may occur. Therefore, analysis and control of said stabilizer additives or stabilizer additive mixtures is a complex task.
Kuznetsov et al. compares the influence of different stabilizer components and additives including K3Fe(CN)6, mercaptobenzthiazol and 4-benzoylpyridin in a formaldehyde and EDTA based electroless copper electrolyte by a chrono-potentiometric method (Surface and Coatings Technology, 28 (1996) 151-160). This method is a currentless method following the potential-time evolution at mixed potential. Solely, discussed is the influence of different additives on the induction period before the autocatalytic copper deposition. However, no concentration dependence of the various additives on the chrono-potentiometric signals was shown.
Paunovic describes a modified chrono-potentiometric method and its application to an electroless copper electrolyte with formaldehyde as reducing agent (J. Electrochem. Soc., 127 (1980) 365-369). A change of the electrode potential at a constant applied current is recorded as a function of time. This method can be used by applying a cathodic as well as anodic constant current. Applying a cathodic current the overpotential shifts into cathodic direction with time until electrolyte decomposition starts due to depletion of copper ions. Applying an anodic current the overpotential shifts into anodic direction with time due to depletion of formaldehyde molecules. This time period between two constant potentials, called transition time, depends on the applied current density. If adsorption reduces the redox active electrode area and leads consequently to an increase of current density, the transition time decreases. This dependence can be used to determine the concentration of surface active additives. This investigation reveals an influence of the concentration of various stabilizer additives, e.g., mercaptobenzothiazole and NaCN on the transition time in cathodic and anodic chrono-potentiometry.
Vitkavage and Paunovic (Annual Technical Conference Proceedings-American Electroplaters' Society (1992), 69th(1), paper A-5, pp. 1-26) investigated the influence of various additive concentrations on copper reduction in an EDTA based electroless copper electrolyte. The authors revealed that with increasing additive concentrations the copper reduction current decreases, depending on the surface activities of the additives and various stirring conditions. This observation is also true for cyanide components as stabilizers. The applied potential run was applied without establishing stationary surface conditions prior the run.
Results from electrochemical impedance spectroscopy (EIS) and coulostatic measurements of an EDTA based electroless copper electrolyte are presented by Sato and Suzuki (J. Electrochem. Soc. 135 (1988) 1645-1650). The concentration of the stabilizer additive 2-mercaptobenzothiazole on platinum electrodes was determined by evaluating the double layer capacities as well as polarization resistances. The polarization resistance arises from electroless formaldehyde oxidation reduced by oxygen. With decreasing additive concentration decreases the polarization resistance. During all measurements no copper deposition occurred.
Rothstein describes a method to determine different additives in various plating electrolytes including mercaptobenzothiazole in an electroless copper electrolyte by square wave voltammetry (M. L. Rothstein, Metal Finishing 1984, October issue, 35-39). A squarewave is superimposed on the linear potential staircase sweep. The current is measured at the end of each half-wave, just prior to potential change. The reduction or oxidation currents of additives is measured directly without preadsorbing steps. The additives have to be oxidized or reduced itself.
The U.S. Pat. No. 4,814,197 discloses methods of analyzing and controlling an electroless plating solution comprising formaldehyde as the reducing agent. The methods also include a procedure for monitoring cyanide ions as stabilizer additive with a cyanide ion sensitive electrode wherein the potential between said CN−-sensitive electrode and a Ag/AgCl reference electrode is measured. Such methods fail in the presence of a reducing agent like formaldehyde (see Example 4 of this invention).
A cyclic voltammtry study (A. M. T. van der Putten, J.-W. G. de Bakker, J. Electrochem. Soc., Vol. 140, No. 8, 1993, 2229-2235) describes the influence of Pb2+ and thiourea stabilizer additives in an electroless nickel bath with hypophosphite as reducing agent on anisotropic nickel plating (bevel plating). The study does not mention attempts to extract the concentration of said stabilizer additives from the corresponding measurements.
The European patent application EP 0 265 901 A2 discloses a method for analyzing an electroless plating solution. The method utilizes a cyanide sensing electrode to determine the cyanide ion stabilizer concentration and a voltammetric method to determine the concentration of other stabilizer agents. The voltammetric method for measuring stabilizer concentrations comprises the steps of a) electrically float and equilibrate the electrodes to assume the mixed potential Emix and b) apply a positive sweep potential increasing the measured potential to a value above Emix. From step b) concentration data of stabilizers are determined by measuring the shift of the peak potential of the plating bath in comparison to a defined reference standard potential, i.e., with the assumption that the peak plating bath potential is a function of the stabilizer agent(s) concentration. By this method only the potential peak position is used to determine the stabilizer concentration without taking into account that the stabilizer concentration is a function of the potential peak position and the current height at the potential peak position. Therefore two parameters are changed in parallel which are not independent of each other and whereas only one parameter is analyzed.
However, by using a combination of both potential peak position and current height at potential peak position a much more accurate determination of the stabilizer concentration can be derived.
In addition the use of a cyanide sensing electrode for measuring the cyanide ion stabilizer concentration in a chemical plating bath does not lead to reproducible results (see Example 4 of the current application).
The patent application US 2003/0201191 A1 discloses a voltammetric method for measuring the concentration of additives in a plating solution wherein the additive concentration is obtained by a ratio of the stripping peak area from the profile of the anodic current to a stripping peak area of a base solution.
The patent document JP 53009235 discloses an electrochemical method for determining the metal ion concentration in an electroless copper coating solution. The potential of the working electrode is changed periodically in this method.
The patent document JP 53009233 discloses an electrochemical method for control of copper electroless coating solution concentration wherein the current of a working electrode is changed periodically.
Thus, there is still a need for a reliable method which allows the measurement and control of a stabilizer additive or mixtures of stabilizer additives in electroless plating electrolytes, especially during use of said plating electrolytes.